1. Field of the Invention
The invention relates to a fuel injection system for internal combustion engines, in particular for direct-injection internal combustion engines, which compared to conventional fuel injection systems has reduced pollutant emissions. Fuel injection systems of this kind can be used particularly for internal combustion engines in the automotive industry, in the field of utility vehicles.
2. Prior Art Description of the Prior Art
In internal combustion engine technology, direct-injection internal combustion engines are being used increasingly, particularly in the field of diesel engines. In them, fuel from a fuel tank is fed via a high-pressure pump system into a high-pressure reservoir (common rail), which in turn is in communication with one or more fuel injectors for injecting the fuel into the combustion chamber of the engine.
As exhaust gas standards become increasingly stringent and given the constant attempt to reduce environmental pollution, numerous systems for posttreating exhaust gases from these internal combustion engines are already being used. Examples that can be mentioned in particular are particle filters, catalytic converters, or combinations of such systems. These systems for posttreating the exhaust gases, however, are often linked with the disadvantage that regular regeneration of the systems is necessary, in order to remove particles, especially soot particles, that have collected in these systems. This regeneration can be done for instance by means of brief temperature elevations, in which particles in the exhaust gas cleaning systems are oxidized and thus removed from the exhaust gas cleaning systems. Typically, stored NOx is reduced by means of uncombusted hydrocarbons.
A known method in the prior art for posttreating exhaust gases is disclosed in German Patent Disclosure DE 198 30 275 A1. This provides a device for posttreating exhaust gases, in the form of a reducing catalytic converter for reducing NOx components of an exhaust gas stream. Diesel fuel is used as the reducing agent; through a special injection nozzle disposed in the exhaust gas tube, it is introduced in such a way that it is aimed in a thin stream at the opening of the outlet valve that opens after a cycle of the engine. By “cracking” and evaporation, the fuel thus introduced is prepared as a reducing agent for reducing the NOx components of the exhaust gas that are stored in the reducing catalytic converter.
The method disclosed in DE 198 30 275 A1 for reinforcing exhaust gas preparation is one example of a so-called postinjection method. Such injection concepts with postinjections to support exhaust gas preparation and/or the regeneration of the systems for exhaust gas preparation, however, have the disadvantage that the postinjection must be controlled chronologically extremely precisely. The postinjection is effected either through the fuel injection valve itself or, as in the case of DE 198 30 275 A1, through a special injection nozzle.
However, if the postinjection quantity is introduced into the engine too soon, it already combusts in the engine, so that on the one hand the exhaust gas temperature is affected in an unwanted way, and on the other, the desired supporting and/or preparing effect of the system is lost for the prose of exhaust gas preparation. If an injection is too late, conversely, the desired cracking effect does not ensue, and thus the postinjection quantity strikes the cylinder wall, for instance, and rinses off the film of lubricant located there and causes the lubricating oil to be diluted with diesel fuel. Thus if injection is too early, the exhaust gas is worsened, and if a postinjection is too late, there is increased wear in the engine because of poorer lubrication. In an extreme case, this latter factor can cause so-called “racing” of the engine from uncontrolled combustion of lubricating oil containing a large amount of diesel fuel.
A remedy to these problems could be provided by making the postinjection not into the combustion chamber but rather directly into the hot exhaust system of the engine. In such concepts, work is done by way of a separate low-pressure injection valve preceded by an electrically actuated metering pump. Such systems are also known as HCI systems (for hydrocarbon injection). With such engines, the possibility also exists of making do in the engine control unit with single-bank injection stage concepts rather than two-bank injection stage concepts, since the late postinjections via the injectors into the engine are omitted. Thus the costs for both electronics and materials are less, and a power loss of the engine control unit is reduced. In addition, the quantity required from the high-pressure pump is reduced, thus making it possible to use smaller pumps as well, in particular single-plunger pumps. The power losses from the requisite generation of high pressure are accordingly less.
Nevertheless, such HCI systems have the disadvantage of requiring complicated additional components. In particular, a separate low-pressure injection valve preceded by an electrically actuated metering pump is necessary. The use of such complicated metering pumps with corresponding electrical control is a particular factor making such HCI systems expensive in terms of space and cost.